The World of LF


Trying a 136kHz TX loop in GM.

Many of the "part 5" permit holders in the states have had great success using large loop aerials after the pioneering work of Bill Ashlock and other "part 15" experimenters on 185kHz. With the high power levels required to get anywhere near 1W erp, currents and voltages get pretty high and the tuning C must withstand it all. I didn't have any suitable Cs in the junk box so decided to make one up from a combination of tried and tested polypropylene Cs and a vacuum variable to tune it over the band. I made the tuner at home so it was completely untested when I got there. Would it withstand the 1kW that my trusty portable TX will produce?

the cs

The 99 Cs (well there's 100 because one of the 1nF is made of 2x2nF)

Arriving in Scotland the beautiful spring weather had given way to rain and high winds (typical!) despite this I managed to get two arrows over the trees, hauled up some cord, then the coax which was to form the loop. I chose the trees to give a N/S orientation with a top span of about 100ft. At one end it was about 60ft high and the other just over 50 although the top was horizontal (ground higher at the South end). To get the right circumference of about 330ft the bottom span was longer than the top and was about 4 to 5ft high.

the loop

The path of the loop (it goes behind the house)

The tuner was placed in the centre of the bottom span standing on an upturned stacking box (so 2ft off the ground). The 12nF required to tune the loop was made up of 99 1nF 2kV polypropylene "pulse rated" Cs connected in 33 parallel legs each having 3 series Cs (333pF each leg). That gives almost 11nF, across this was a 2nF vacuum variable C motor-driven from the shack.

feed point of the loop

The tuner at the centre of the bottom run.

The transformer in the box was made of 3 stacked 58mm 3C85 toroids with 20 turns on the primary and two turns of huge fat "starter motor" type cable as the secondary in series with the loop and Cs. The transformer remained completely stone cold even at 1kW input. I think I over-engineered it a bit after some tales of over heating from the states!

On connecting the loop to the feeder and receiver the band sounded very lively and the German signal on 138.83 was S9+ so my calculations can't have been far off. A test with the DDS revealed the loop was resonant on 143kHz and the variable C was hardly meshed. It tuned very slowly thanks to the reduction drive built into the vac variable but eventually it arrived on 137kHz with more C available. I connected the portable TX to the 12V PSU, no danger of blowing anything up with a max input of 200W! Then I put my 6A current meter in series with the loop and connected up the TX via my HF SWR bridge. On 137kHz the 6A meter hit the endstop with only about 50W of RF, the SWR looked pretty good too.. things were going well. Time to set up the 1kW TX!

the tuner box

Close-up of the tuner box.

Not so good now.. the big TX has very effective SWR protection and the 2:1 match that the loop was giving was too much for it. Of course I could have disabled it, but this TX has never blown up and I think that's mostly due to its protection circuits. I could play with the transformer in the box but I didn't want to keep running in and out of the house, what I needed was a matching network.

Hunting in the "radio box", a stacking box full of almost everything I could possibly need and a lot of things that I never will, I found two more 58mm ferrite toroids and some 1.5mm wire. On the stacked cores I wound about 25 turns and tapped the last 6 or so so that I could step up or down a bit depending which way round I used it. In the event it needed a step-up to get a 1:1 match and tests revealed no heating in the transformers or the Cs at about 600W which was as much as I dared run to start with.


Current in the loop was calculated at over 30A so I decided to measure it. I wound about 3ft of wire around a piece of cardboard which I wrapped around the loop wire and taped up. I found a diode and a C in the radio box and connected them to the winding on the cardboard current transformer. I put the current meter back in the loop and adjusted the TX to give enough RF to make the 6A meter read 5A, the voltage coming from the transformer was 1.2V. I removed the 6A meter and turned the power up, I got a maximum of 8V. That's 6.66 times 1.2V so we had about 33A in the loop

This means each C was passing around 1A. I later found that the frequency did move slightly HF under long periods of QRSS and the heating of the Cs could be felt by hand but they were by no means hot. I guess they have a negative temperature coefficient?

Over the four days I worked 7 countries with "O" reports including OH1TN off the side of the loop and a very nice QSO with EA1PX who would have been right on my "beam". I did try to work RU6LA, who was a reasonable copy on the loop despite being in the wrong direction. He did get some signals but we failed in the end.

the shack

"Shack" with Marconi loading coil extreme left, TX, TS2000 with DDS on top, washing machine etc.

Later, when the weather improved I got out the trusty bow and arrow again and put up a nice Marconi inverted L with a 120ft top at 65ft high and a 45 degree downlead of about 90ft. Due to the poor ground and proximity to trees I could only get 2.5A up this wire with 1kW, my best efforts on this site in the past have given over 3A (more top wires, more radials). Nevertheless the Marconi gave exactly the same signal reports with G3NYK, G3KEV and G3AQC with whom I did tests. It was, however, much noisier on receive. Every station I heard, regardless of direction, was better copy on the loop.

I also took the PA0RDT mini-whip with me and on a 20ft pole it was easily as sensitive as the big aerials, slightly less noisy than the Marconi but not as quiet as the loop.

So the loop was easy to erect, no worries about wires touching branches, robust due to the thick cable, performed as well as a similar Marconi on TX and was quiet on receive. In the end I was able to run the TX at full power with no ill effects apart from the correctable "drift" as the Cs heated. On the negative side the lower element gets in the way and it required a specialist tuner... but now I've got one I could be using loops more often! I've left both aerials up for the next visit but I know which one is more likely to withstand the gales...


WD2XES loop info
The KL1X loop tuner box

More tales